U.S. patent application number 13/051894 was filed with the patent office on 2012-09-20 for encapsulant with index matched thixotropic agent.
This patent application is currently assigned to CREE, Inc.. Invention is credited to BERND KELLER, Theodore Lowes.
Application Number | 20120235190 13/051894 |
Document ID | / |
Family ID | 46827783 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120235190 |
Kind Code |
A1 |
KELLER; BERND ; et
al. |
September 20, 2012 |
ENCAPSULANT WITH INDEX MATCHED THIXOTROPIC AGENT
Abstract
Emitter packages are disclosed having a thixotropic agent or
material, with the encapsulant exhibiting significant reduction of
thixotropic agent scattering. The packages exhibit a corresponding
reduction or elimination of encapsulant clouding and increased
package emission efficiency. This allows for the thixotropic agents
to be included in the encapsulant to alter certain properties (e.g.
mechanical or thermal) while not significantly altering the optical
properties of the encapsulant. One embodiment of a light emitting
diode (LED) package according to the present invention comprises an
LED chip with an encapsulant over the LED chip. The encapsulant has
an encapsulant refractive index and also has a thixotropic material
with a refractive index that is substantially the same as the
encapsulant refractive index.
Inventors: |
KELLER; BERND; (Santa
Barbara, CA) ; Lowes; Theodore; (Lompoc, CA) |
Assignee: |
CREE, Inc.
|
Family ID: |
46827783 |
Appl. No.: |
13/051894 |
Filed: |
March 18, 2011 |
Current U.S.
Class: |
257/98 ;
257/E33.073 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 33/56 20130101; H01L 2924/00 20130101; H01L 2924/0002
20130101 |
Class at
Publication: |
257/98 ;
257/E33.073 |
International
Class: |
H01L 33/52 20100101
H01L033/52 |
Claims
1. A light emitting diode (LED) package, comprising: an LED chip;
an encapsulant over said LED chip, said encapsulant having a
encapsulant refractive index; and a thickening material disposed
therein, said material comprising a composite of two or more
materials at least one of which has an index of refraction lower
than said encapsulant refractive index and at least one of which
has a material with a refractive index higher than said encapsulant
refractive index.
2. The LED package of claim 1, wherein said thickening material
comprises a thixotropic material.
3. The LED package of claim 1, wherein said LED chip emits LED
light that passes through said encapsulant, said encapsulant and
thickening material combination being substantially non-scattering
to said LED light.
4. The LED package of claim 1, wherein said thickening material has
a refractive index that is plus or minus n=0.05 of the encapsulant
refractive index.
5. The LED package of claim 1, wherein said thickening material has
a refractive index that is plus or minus n=0.04 of the encapsulant
refractive index.
6. The LED package of claim 1, wherein said thickening material has
a refractive index that is plus or minus n=0.02 of the encapsulant
refractive index.
7. The LED package of claim 1, wherein said thickening material
comprises particles with a surface area greater than 100 meters
squared per gram (m.sup.2/g).
8. The LED package of claim 1, wherein said thickening material
comprises particles with a surface area greater than 300 meters
squared per gram (m.sup.2/g).
9. The LED package of claim 1, wherein said thickening material
comprises particles with a size less than 0.1 micrometers
(.mu.m).
10. The LED package of claim 1, wherein said thickening material
comprises particles with a size less than 0.5 micrometers
(.mu.m).
11. The LED package of claim 1, wherein said encapsulant comprises
a material with refractive index of n.apprxeq.1.51, and said
thickening material refractive index n.apprxeq.1.51, plus or minus
0.05.
12. The LED package of claim 1, wherein said thickening material
comprises a one or more oxides.
13. The LED package of claim 1, wherein said encapsulant comprises
a polymer and said thickening material comprises on or more
oxides.
14. The LED package of claim 1, wherein said encapsulant comprises
silicone, and said thickening material comprises an aluminosilicate
composite.
15. The LED package of claim 1, wherein said thickening material
comprises a combination of a material with higher index of
refraction than said encapsulant, with a material having a lower
index of refraction than said encapsulant.
16. The LED package of claim 1, wherein said thickening material
alters the mechanical properties of said encapsulant without
substantially altering its optical properties.
17. The LED package of claim 1, wherein said thickening material
alters the thermal properties of said encapsulant without
substantially altering its optical properties.
18. The LED package of claim 1, wherein said thickening material
alters the electrical properties of said encapsulant without
substantially altering its optical properties.
19. The LED package of claim 1, further comprising a submount, said
LED mounted to said submount.
20. A light emitting diode (LED) package, comprising: an LED chip
emitting light in response to an electrical signal; and an
encapsulant over said LED chip, at least some of said LED light
passing through said encapsulant, wherein said encapsulant has a
thixotropic agent dispersed therein, with the encapsulant and
thixotropic agent combination being substantially optically
non-scatting to said LED light.
21. A light emitting diode (LED) package, comprising: an LED chip;
and an encapsulant over said LED chip, said encapsulant having a
encapsulant refractive index and having a thickening material with
a refractive index that is within plus or minus n=0.05 of the
encapsulant refractive index.
22. The LED package of claim 21, wherein said thickening material
comprises a thixotropic material.
23. A light emitting diode (LED) package, comprising: an LED chip;
and an encapsulant over said LED chip, said encapsulant having an
encapsulant refractive index and having a thixotropic material with
a refractive index that is approximately the same as said
encapsulant refractive index.
24. The LED package of claim 23, wherein said thixotropic material
has a refractive index that is plus or minus n=0.05 of the
encapsulant refractive index.
25. The LED package of claim 23, wherein said thixotropic material
has a refractive index that is plus or minus n=0.04 of the
encapsulant refractive index.
26. The LED package of claim 24, wherein said thixotropic material
has a refractive index that is plus or minus n=0.02 of the
encapsulant refractive index.
27. The LED package of claim 19, wherein said composite thixotropic
material has comprises silica and alumina.
28. A light emitting diode (LED) package, comprising: an LED chip;
and an encapsulant over said LED chip, said encapsulant having a
thickening material disposed therein, said thixotropic agent
altering the mechanical properties of said encapsulant compared to
the same encapsulant without said thickening material, without
substantially altering the optical properties of said
encapsulant.
29. The LED package of claim 28, wherein said thickening material
comprises a thixotropic material.
30. The LED package of claim 29, wherein said encapsulant and
thixotropic material combination are substantially clear to light
emitted by said LED.
31. A light emitting diode (LED) package, comprising: an LED chip;
and an encapsulant over said LED chip, said encapsulant having a
thickening material disposed therein, said thixotropic agent
altering the thermal properties of said encapsulant compared to the
same encapsulant without said thickening material, without
substantially altering the optical properties of said
encapsulant.
32. The LED package of claim 28, wherein said thickening material
comprises a thixotropic material.
33. The LED package of claim 32, wherein said thixotropic material
increases the thermal conductance of said encapsulant compared to
the same encapsulant without said thixotropic material.
34. The LED package of claim 31, wherein said encapsulant and
thixotropic material combination are substantially clear to light
emitted by said LED.
35. A light emitting diode (LED) package, comprising: an LED chip;
an encapsulant over said LED chip, said encapsulant having a
encapsulant refractive index; and a thickening material disposed
therein, said material comprising a composite of two or more oxide
materials, said composite material having a refractive index that
is within plus or minus n=0.05 of the encapsulant refractive
index.
36. The LED package of claim 35, wherein said thickening material
comprises a thixotropic material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to solid state lighting packages, and
more particularly to LED packages having encapsulants with
thixotropic agents.
[0003] 2. Description of the Related Art
[0004] Incandescent or filament-based lamps or bulbs are commonly
used as light sources for both residential and commercial
facilities. However, such lamps are highly inefficient light
sources, with as much as 95% of the input energy lost, primarily in
the form of heat or infrared energy. One common alternative to
incandescent lamps, so-called compact fluorescent lamps (CFLs), are
more effective at converting electricity into light but require the
use of toxic materials which, along with its various compounds, can
cause both chronic and acute poisoning and can lead to
environmental pollution. One solution for improving the efficiency
of lamps or bulbs is to use solid state devices such as light
emitting diodes (LED or LEDs), rather than metal filaments, to
produce light.
[0005] Light emitting diodes generally comprise one or more active
layers of semiconductor material sandwiched between oppositely
doped layers. When a bias is applied across the doped layers, holes
and electrons are injected into the active layer where they
recombine to generate light. Light is emitted from the active layer
and from various surfaces of the LED.
[0006] In order to use an LED chip in a circuit or other like
arrangement, it is known to enclose an LED chip in a package to
provide environmental and/or mechanical protection, color
selection, light focusing and the like. An LED package also
includes electrical leads, contacts or traces for electrically
connecting the LED package to an external circuit. In a typical LED
package 10 illustrated in FIG. 1, a single LED chip 12 is mounted
on a reflective cup 13 by means of a solder bond or conductive
epoxy. One or more wire bonds 11 connect the ohmic contacts of the
LED chip 12 to leads 15A and/or 15B, which may be attached to or
integral with the reflective cup 13. The reflective cup may be
filled with an encapsulant material 16 which may contain a
wavelength conversion material such as a phosphor. Light emitted by
the LED at a first wavelength may be absorbed by the phosphor,
which may responsively emit light at a second wavelength. The
entire assembly can then be encapsulated in a clear protective
resin 14, which may be molded in the shape of a lens to collimate
the light emitted from the LED chip 12.
[0007] FIG. 2 shows another embodiment of a conventional LED
package 20 comprising one or more LED chips 22 mounted to a carrier
such as a printed circuit board (PCB) carrier, substrate or
submount 23. A metal reflective cup 24 mounted on the submount 23
surrounds the LED chip(s) 22 and reflects light emitted by the LED
chips 22 away from the package 20. The reflective cup 24 also
provides mechanical protection to the LED chips 22. One or more
wire bond connections 27 are made between ohmic contacts on the LED
chips 22 and electrical traces 25A, 25B on the submount 23. The
mounted LED chips 22 are then covered with an encapsulant 26, which
may provide environmental and mechanical protection to the chips
while also acting as a lens. The metal reflective cup 24 is
typically attached to the carrier by means of a solder or epoxy
bond.
[0008] FIG. 3 shows another embodiment conventional LED package 30
having an LED chip 32 mounted on a submount 34, similar to the LED
package 20 shown in FIG. 2. In this embodiment, however, there is
no reflective cup. In this embodiment, an encapsulant 36 is formed
directly over the LED chip 32 and on the surface of the submount
around the LED chip 32. Like the encapsulant in package 20, the
encapsulant 36 can provide environmental and/or mechanical
protection and can shape or alter the light emitting from the
package.
[0009] Many encapsulants used in conventional LED packages utilize
a thixotropic or thickening agent/material that can help the
encapsulant maintain the desired shape. The packages shown in FIGS.
2 and 3, the encapsulant is in a hemispheric shape, and for
dispensed lenses the thixotropic agent helps the encapsulant
maintain a hemispheric shape particularly in the time between when
the encapsulant is dispensed or molded, and when it is cured. One
of the most common thixotropic agents is fumed silica that is
commercially available from sources such as Cabot Corporation and
Evonik Industries. Fumed silica is a relatively common material
that is used in many different applications and has a very strong
thickening effect. Fumed silica can be provided in particles that
typically have a size in the range of 5-50 nanometers (nm). The
particles can be non-porous and can have a surface area of 50-600
m.sup.2/g, with a density of around 2.2 g/cm.sup.3.
[0010] Many thixotropic agents can have an index of refraction that
is different from the LED package encapsulant. For example, fumed
silica can have an index of refraction of approximately 1.46, and
can be mixed in a conventional encapsulant such as silicone which
has an index or refraction of 1.51 or more. This difference in
index of refraction between the thixotropic agent and the
encapsulant can result in the encapsulant exhibiting scattering
characteristics for the light passing through the encapsulant from
the LED. This scattering not only gives the encapsulant a cloudy
(i.e. not clear) appearance, but can reduce emission package
efficiency by reducing the total light output from the package.
SUMMARY OF THE INVENTION
[0011] The present invention is generally directed to emitter
packages having a thickening or thixotropic agent/material, with
properties that allow the LED package encapsulant to exhibit a
significant reduction in scattering. The packages according to the
present invention exhibit a corresponding reduction or elimination
of encapsulant clouding and increased package emission efficiency.
This allows for the thickening or thixotropic agents to alter
certain properties (e.g. mechanical or thermal) while not
significantly altering the optical properties of the
encapsulant.
[0012] One embodiment of a light emitting diode (LED) package
according to the present invention comprises a LED chip and an
encapsulant over the LED chip, with the encapsulant having an
encapsulant refractive index. A thickening (or thixotropic)
material is disposed in the encapsulant with the material
comprising a composite of two or more materials, at least one of
which has an index of refraction lower than the encapsulant
refractive index, and at least one of which has a material with a
refractive index higher than the encapsulant refractive index.
[0013] Another embodiment of an LED package according to the
present invention comprises an LED chip emitting light in response
to an electrical signal. An encapsulant is included over the LED
chip with at least some of the LED light passing through the
encapsulant. The encapsulant has a thickening agent dispersed
therein with the encapsulant and thickening agent combination being
substantially optically clear to the LED light.
[0014] Another embodiment of an LED package according to the
present invention comprises an LED chip and an encapsulant over the
LED chip. The encapsulant has an encapsulant refractive index and
has a thixotropic material with a refractive index that is within
plus or minus n=0.05 of the encapsulant refractive index.
[0015] One embodiment of a light emitting diode (LED) package
according to the present invention comprises a LED chip with an
encapsulant over the LED chip. The encapsulant has an encapsulant
refractive index and also has a thixotropic material with a
refractive index that is approximately the same as the encapsulant
refractive index.
[0016] These and other aspects and advantages of the invention will
become apparent from the following detailed description and the
accompanying drawings which illustrate by way of example the
features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a sectional view of one embodiment of a
conventional LED package;
[0018] FIG. 2 is a sectional view of another embodiment of a
conventional LED package;
[0019] FIG. 3 is a sectional view of still another embodiment of a
conventional LED package;
[0020] FIG. 4 is a sectional view of two LED packages according to
the present invention prior to singulating;
[0021] FIG. 5 is a sectional view of the LED packages in FIG. 4
following singulating;
[0022] FIG. 6 is a sectional view of another embodiment of an LED
package according to the present invention having encapsulant
portions;
[0023] FIG. 7 is a sectional view of another embodiment of an LED
package according to the present invention having encapsulant
portions;
[0024] FIG. 8 is a sectional view of another embodiment of an LED
package according to the present invention having encapsulant
portions;
[0025] FIG. 9 is a sectional view of still another embodiment of an
LED package according to the present invention having encapsulant
portions;
[0026] FIG. 10 is a top view of one embodiment of an LED package
encapsulant according to the present invention; and
[0027] FIG. 11 is a top view of another embodiment of an LED
package encapsulant according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention is directed to different embodiments
of semiconductor device packages, and in particular solid state
lighting packages utilizing an encapsulant. Some embodiments of the
present invention are directed to light emitting diode (LED)
packages having an encapsulant for environmental and/or mechanical
protection, color selection, light focusing and the like. The
encapsulant can comprise a thickening or thixotropic agent/material
that helps hold the encapsulant in the desired shape prior to and
during curing.
[0029] As mentioned above, it can be desirable to include a
thickening agent or material in the encapsulant to help it hold the
desired shape (e.g. hemispheric) prior to and during curing.
Thickening can also include increasing the viscosity of the
encapsulant. In the case of encapsulants that are formed by
processes such as stamping or molding, many different thickening
materials can be used. For encapsulants that are formed by
dispensing it can be desirable to have thickening agents that
exhibit thixotropic properties. Thixotropic agents comprise a
material, such as particles, that can be mixed in the encapsulant
and thixotropic properties include a thickening effect when the
encapsulant material is at rest. In some embodiments this
thickening can form the encapsulant in a gel state. Thixotropic
properties can also include the characteristics of encapsulant
material becoming a liquid when agitated, stirred or shaken. It can
be difficult dispensing encapsulant materials that are not in
liquid form, and this property of becoming liquid when agitated can
be desirable for dispensing encapsulants. The thixotropic
properties allow for the encapsulant material to be in a liquid
form, such as through agitation, prior to and as the encapsulant
material is being dispensed over the LED(s). The thixotropic
properties then allow the encapsulant to take its thickened state
over the LED when the encapsulant comes to rest over the LED, and
allows the encapsulant to hold the desired shape until cured.
[0030] The discussion below refers to thixotropic agents or
materials, but it is understood that this also refers to thickening
agents that do not exhibit thixotropic characteristics. These
non-thixotropic thickening agents can be utilized in molding and
stamping processes for forming the encapsulant. It is understood
that the invention described herein encompasses thickening
materials that can be thixotropic and non-thixotropic agents or
materials.
[0031] The different embodiments of the present invention comprise
thixotropic agents that comprise a material having an index of
refraction that is the same as or close to that of the encapsulant
material. By matching the index of refraction, the scattering
effect of the thixotropic agent can be reduced or eliminated.
Stated differently, the light from the package LED or LEDs can pass
through the encapsulant without being scattered interfaces between
the thixotropic materials and the encapsulant materials of
different refractive indices. The light can pass through the
encapsulant in approximately the same manner as light passing
through an encapsulant without a thixotropic agent. By not
scattering the light, the cloudy appearance of the encapsulant can
be reduced or eliminated, and the decrease in LED package emission
caused by the scattering can be reduced or eliminated.
[0032] The present invention is directed to packages having many
different types of encapsulants that can comprise a single material
or combinations of materials. Likewise, the thixotropic agents can
comprise many different materials that when combined in the
appropriate percentages result in the desired refractive index that
matches or nearly matches that of the encapsulant. In some
embodiments the encapsulant can comprise a polymer material, and
the thixotropic agent can comprise one or more oxides or ceramics.
In still other embodiments, the encapsulant can comprise other
materials mentioned below, with the thixotropic agent comprising a
combination of one or more oxides or ceramics.
[0033] Matching the refractive index of the encapsulant and
thixotropic agent can allow for changing certain properties of the
encapsulant without changing the optical characteristics of the
encapsulant. In some embodiments, the thixotropic agent can alter
the mechanical properties of the encapsulant, without altering the
encapsulant's optical properties. These mechanical properties can
include, but are not limited to, the thickening effect mentioned
above that allows an encapsulant to hold its shape prior to curing,
or the durometer of the cured lens (a measure of the hardness of
the cured material).
[0034] This matching of refractive index can also allow for
increased percentages of thixotropic agents to be included in the
encapsulant. That is, because the thixotropic agents do not scatter
light, or results in minimal scattering, increased percentages of
thixotropic agents can be included in the encapsulant. This can
allow for changing of the mechanical properties to a greater degree
without unacceptable impact on the package's optical
properties.
[0035] In other embodiments, other properties can be changed
without changing optical properties. Thixotropic agents that have
an increased thermally conductivity, yet are index matched, can be
added to the encapsulant to enhance its thermal dissipation
characteristics. By being index matched, increased percentages of
the thixotropic agents can be added without a negative impact on
appearance and emission efficiency, while at the same time
increasing the ability of the encapsulant to dissipate heat from
the LED or LEDs. In these embodiments, both the mechanical (i.e.
thickening) and thermal properties of the encapsulant can be
enhanced. In these embodiments, the encapsulant can also be
provided with features to further enhance light extraction, such as
roughness, protrusions or heat fins.
[0036] In those embodiments where it can be desirable to increase
the electrical conductivity of the encapsulant, electrically
conductive thixotropic agents can be included in the encapsulant.
Like above, these agents can be index matched to encapsulant such
that increased percentages of the agents can be included without
negatively impacting the emission properties of the package. In
these embodiments, both the mechanical and thermal properties of
the encapsulant can be enhanced. In still other embodiments, the
mechanical, thermal and electrical properties can all be enhanced
by providing the appropriate thixotripic agent index matched to the
encapsulant, and being electrically and thermally conductive.
[0037] The present invention is described herein with reference to
certain embodiments, but it is understood that the invention can be
embodied in many different forms and should not be construed as
limited to the embodiments set forth herein. In particular, the
present invention is described below in regards to certain LED
packages having one or multiple LEDs or LED chips in different
configurations, but it is understood that the present invention can
be used for many other lamps having many different configurations.
The embodiments below are described with reference to LED of LEDs,
but it is understood that this is meant to encompass LED chips and
LED packages. The components can have different shapes and sizes
beyond those shown and different numbers of LEDs can be included.
Similarly, in some embodiment, the LED or LEDs can be coated by
phosphor layers or regions, while others may have either adjacent
phosphor layers of different composition or no phosphor layer at
all.
[0038] The present invention is described herein with reference to
LED package encapsulants deposited on or over the LED(s), but it is
understood that this can include lens or lens materials and can
comprise lenses that are formed separately and then mounted over
the LED. Further, this description is meant to include encapsulants
that directly contact the LED, and those that do not. An opening
can be included between the LED chip and the encapsulant, or an
intervening layer or material can be between the two.
[0039] The present invention is described herein with reference to
conversion materials, thixotropic or thickening agents, wavelength
conversion materials, remote phosphors, phosphors, phosphor layers
and related terms. The use of these terms should not be construed
as limiting. It is understood that many different thixotropic or
thickening agents can be used and the term remote phosphors,
phosphor or phosphor layers is meant to encompass and be equally
applicable to all wavelength conversion materials.
[0040] It is also understood that when an element such as a layer,
region or substrate is referred to as being "on" another element,
it can be directly on the other element or intervening elements may
also be present. Furthermore, relative terms such as "inner",
"outer", "upper", "above", "lower", "beneath", and "below", and
similar terms, may be used herein to describe a relationship of one
layer or another region. It is understood that these terms are
intended to encompass different orientations of the device in
addition to the orientation depicted in the figures.
[0041] Although the terms first, second, etc. may be used herein to
describe various elements, components, regions, layers and/or
sections, these elements, components, regions, layers and/or
sections should not be limited by these terms. These terms are only
used to distinguish one element, component, region, layer or
section from another region, layer or section. Thus, a first
element, component, region, layer or section discussed below could
be termed a second element, component, region, layer or section
without departing from the teachings of the present invention.
[0042] Embodiments of the invention are described herein with
reference to cross-sectional view illustrations that are schematic
illustrations of embodiments of the invention. As such, the actual
thickness of the layers can be different, and variations from the
shapes of the illustrations as a result, for example, of
manufacturing techniques and/or tolerances are expected.
Embodiments of the invention should not be construed as limited to
the particular shapes of the regions illustrated herein but are to
include deviations in shapes that result, for example, from
manufacturing. A region illustrated or described as square or
rectangular will typically have rounded or curved features due to
normal manufacturing tolerances. Thus, the regions illustrated in
the figures are schematic in nature and their shapes are not
intended to illustrate the precise shape of a region of a device
and are not intended to limit the scope of the invention.
[0043] FIGS. 4 and 5 show one embodiment of LED packages 50
according to the present invention. In FIG. 4, the LED packages 50
are shown prior to separation of the submount 52 into individual
packages. Although only two LED packages 50 are shown, it is
understood that many packages can be formed as part formed together
submount 52, with the submount 52 being cut or etched to separated
portions of the submount 52 to provide the individual packages.
[0044] Each of the LED packages 50 can comprise a portion of the
submount 52 for holding an LED chip 54, with some embodiment of the
submount 52 having a die pad and conductive traces (not shown) on
its top surface. The LED package embodiments shown comprises only a
single LED, but it is understood that other embodiments can
comprise multiple LEDs arranged in arrays in different ways with
different interconnections. Examples of certain LED arrays that can
be incorporated in the LED packages according to the present
invention are described in U.S. Pat. No. 7,821,023 and U.S. Patent
Application Publication No. 2009/0050908, both to Yuan et al., and
both entitled "Solid State Lighting Component", and U.S. Patent
Application Publication No. 2010/0103660 to van de Ven et al. and
entitled "Array Layout for Color Mixing," all three of which are
incorporated herein by reference.
[0045] The LEDs chip 54 can have many different semiconductor
layers arranged in different ways and can emit many different
colors in different embodiments according to the present invention.
LED structures, features, and their fabrication and operation are
generally known in the art and only briefly discussed herein.
[0046] The layers of the LED chip 54 can be fabricated using known
processes with a suitable process being fabrication using metal
organic chemical vapor deposition (MOCVD). The layers of the LED
chips generally comprise an active layer/region sandwiched between
first and second oppositely doped epitaxial layers all of which are
formed successively on a growth substrate. LED chips can be formed
on a wafer and then singulated for mounting in a package. It is
understood that the growth substrate can remain as part of the
final singulated LED or the growth substrate can be fully or
partially removed.
[0047] It is also understood that additional layers and elements
can also be included in the LED chip 54, including but not limited
to buffer, nucleation, contact and current spreading layers as well
as light extraction layers and elements. The active region can
comprise single quantum well (SQW), multiple quantum well (MQW),
double heterostructure or super lattice structures. The active
region and doped layers may be fabricated from different material
systems, with preferred material systems being Group-III nitride
based material systems. Group-III nitrides refer to those
semiconductor compounds formed between nitrogen and the elements in
the Group III of the periodic table, usually aluminum (Al), gallium
(Ga), and indium (In). The term also refers to ternary and
quaternary compounds such as aluminum gallium nitride (AlGaN) and
aluminum indium gallium nitride (AlInGaN). In a preferred
embodiment, the doped layers are gallium nitride (GaN) and the
active region is InGaN. In alternative embodiments the doped layers
may be AlGaN, aluminum gallium arsenide (AlGaAs) or aluminum
gallium indium arsenide phosphide (AlGaInAsP) or aluminum indium
gallium phosphide (AlInGaP) or zinc oxide (ZnO).
[0048] The growth substrate can be made of many materials such as
silicon, glass, sapphire, silicon carbide, aluminum nitride (AlN),
gallium nitride (GaN), with a suitable substrate being a 4H
polytype of silicon carbide, although other silicon carbide
polytypes can also be used including 3C, 6H and 15R polytypes.
Silicon carbide has certain advantages, such as a closer crystal
lattice match to Group III nitrides than sapphire and results in
Group III nitride films of higher quality. Silicon carbide also has
a very high thermal conductivity so that the total output power of
Group-III nitride devices on silicon carbide is not limited by the
thermal dissipation of the substrate (as may be the case with some
devices formed on sapphire). SiC substrates are available from Cree
Research, Inc., of Durham, North Carolina and methods for producing
them are set forth in the scientific literature as well as in a
U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022.
[0049] The LED chip 54 can also comprise a conductive current
spreading structure and wire bond pads on the top surface, both of
which are made of a conductive material and be deposited using
known methods. Some materials that can be used for these elements
include Au, Cu, Ni, In, Al, Ag or combinations thereof and
conducting oxides and transparent conducting oxides. The current
spreading structure can comprise conductive fingers arranged in a
grid on the LED chip 54 with the fingers spaced to enhance current
spreading from the pads into the LED's top surface. In operation,
an electrical signal is applied to the pads through a wire bond as
described below, and the electrical signal spreads through the
fingers of the current spreading structure and the top surface into
the LED chips 54. Current spreading structures are often used in
LEDs where the top surface is p-type, but can also be used for
n-type materials.
[0050] Some or all of the LED chip 54 can be coated with one or
more phosphors with the phosphors absorbing at least some of the
LED light and emitting a different wavelength of light such that
the LED emits a combination of light from the LED and the phosphor.
As described in detail below, in one embodiment according to the
present invention at least some of the LED chips can comprise an
LED that emits light in the blue wavelength spectrum with its
phosphor absorbing some of the blue light and re-emitting yellow
light. These LED chips 54 emit a white light combination of blue
and yellow light or a non-white light combination of blue and
yellow light. As used herein, the term "white light" refers to
light that is perceived as white and is within 7 MacAdam ellipses
of the black body locus on a 1931 CIE chromaticity diagram and has
a CCT ranging from 2000 K to 10,000 K. In one embodiment the
phosphor comprises commercially available YAG:Ce, although a full
range of broad yellow spectral emission is possible using
conversion particles made of phosphors based on the
(Gd,Y).sub.3(Al,Ga).sub.5O.sub.12:Ce system, such as the
Y.sub.3Al.sub.5O.sub.12:Ce (YAG). Other yellow phosphors that can
be used for white emitting LED chips include:
Tb.sub.3-xRE.sub.xO.sub.12:Ce(TAG); RE=Y, Gd, La, Lu; or
Sr.sub.2-x-yBa.sub.xCa.sub.ySiO.sub.4:Eu.
[0051] In other embodiments, the LED chip can comprise blue
emitting LED coated by other phosphors that absorb blue light and
emit yellow or green light. Some of the phosphors that can be used
for these LED chips include:
YELLOW/GREEN
(Sr,Ca,Ba)(Al,Ga).sub.2S.sub.4:Eu.sup.2+
Ba.sub.2(Mg,Zn)Si.sub.2O.sub.7:Eu.sup.2+
Gd.sub.0.46Sr.sub.0.31Al.sub.1.23O.sub.xF.sub.1.38:EU.sup.2+.sub.0.06
[0052] (Ba.sub.1-x-ySr.sub.xCa.sub.y) SiO.sub.4:Eu
Ba.sub.2SiO.sub.4:Eu.sup.2+
[0053] The packages can also comprise an LED chip emitting red
light, that can comprise LED structures and materials that permit
emission of red light directly from the active region.
Alternatively, in other embodiments a red emitting LED chip 54 can
comprise LEDs covered by a phosphor that absorbs the LED light and
emits a red light. Some phosphors appropriate for this structures
can comprise:
RED
Lu.sub.2O.sub.3:Eu.sup.3+
[0054] (Sr.sub.2-xLa.sub.x)(Ce.sub.1-xEu.sub.x)O.sub.4
Sr.sub.2Ce.sub.1-xEu.sub.xO.sub.4 Sr.sub.2-xEu.sub.xCeO.sub.4
SrTiO.sub.3:Pr.sup.3+,Ga.sup.3+
CaAlSiN.sub.3:Eu.sup.2+
Sr.sub.2Si.sub.5N.sub.8:Eu.sup.2+
[0055] Each of the phosphors described above exhibits excitation in
the desired emission spectrum, provides a desirable peak emission,
has efficient light conversion, and has acceptable Stokes shift. It
is understood, however, that many other phosphors can used in
combination with other LED colors to achieve the desired color of
light.
[0056] LED chip 54 can be coated with a phosphor using many
different methods, with one suitable method being described in U.S.
patent applications Ser. Nos. 11/656,759 and 11/899,790, both
entitled "Wafer Level Phosphor Coating Method and Devices
Fabricated Utilizing Method", and both of which are incorporated
herein by reference. Alternatively the LEDs can be coated using
other methods such as electrophoretic deposition (EPD), with a
suitable EPD method described in U.S. patent application Ser. No.
11/473,089 entitled "Close Loop Electrophoretic Deposition of
Semiconductor Devices", which is also incorporated herein by
reference. It is understood that LED packages according to the
present invention can also have multiple LEDs of different colors,
one or more of which may be white emitting.
[0057] The submount 52 can be formed of many different materials
with a preferred material being electrically insulating, such as a
dielectric. The submount 52 can comprise ceramic such as alumina,
aluminum nitride, silicon carbide, or a polymeric material such as
polyimide and polyester etc. In the preferred embodiment, the
submount material has a high thermal conductivity such as with
aluminum nitride and silicon carbide. In other embodiments the
submount 52 can comprise highly reflective material, such as
reflective ceramic or metal layers like silver, to enhance light
extraction from the component. In other embodiments the submount 52
can comprise a printed circuit board (PCB), sapphire, silicon
carbide or silicon or any other suitable material, such as T-Clad
thermal clad insulated substrate material, available from The
Bergquist Company of Chanhassen, Minn. For PCB embodiments
different PCB types can be used such as standard FR-4 PCB, metal
core PCB, or any other type of printed circuit board. The size of
the submount 52 can vary depending on different factors, with one
being the size and number of LED chips 54.
[0058] The submount 52 can have die pads and conductive traces that
can comprise many different materials such as metals or other
conductive materials. In one embodiment they can comprise copper
deposited using known techniques such as plating and can then be
patterned using standard lithographic processes. In other
embodiments the layer can be sputtered using a mask to form the
desired pattern. In some embodiments according to the present
invention some of the conductive features can include only copper,
with others of the features including additional materials. For
example, the die pads can be plated or coated with additional
metals or materials to the make them more suitable for mounting of
LEDs. In one embodiment the die pads can be plated with adhesive or
bonding materials, or reflective and barrier layers. The LEDs can
be mounted to the die pads using known methods and materials such
as using conventional solder materials that may or may not contain
a flux material or dispensed polymeric materials that may be
thermally and electrically conductive.
[0059] In the embodiment shown, wire bonds can be included that
pass between the conductive traces and the LED chip 54 with an
electrical signal applied to the LED chip 54 through its respective
one of the die pads and the wire bonds. In other embodiments, LED
chips 54 may comprise coplanar electrical contacts on one side of
the LED (bottom side) with the majority of the light emitting
surface being located on the LED side opposing the electrical
contacts (upper side). Such flip-chip LEDs can be mounted onto the
submount 52 by mounting contacts corresponding to one electrode
(anode or cathode, respectively) onto the die pad. The contacts of
the other LED electrode (cathode or anode, respectively) can be
mounted to the traces.
[0060] An encapsulant 56 can be included over the LED chip 54 to
provide both environmental and mechanical protection. The
encapsulant can also act as an optical element or lens over the LED
chips 54. In some embodiments the encapsulant 56 can be formed
directly over and in direct contact with the LED chip 54 and the
top surface of the submount 52 around the LED chips. In other
embodiments there may be an intervening material or layer between
the encapsulant and LED chip 54 and/or the submount's top surface.
Direct contact to the LED chip 54 can provide certain advantages
such as improved light extraction and ease of fabricating.
[0061] As further described below, the encapsulant 56 can be formed
over the LED chips 54 using different encapsulant dispense or
molding techniques and the lens can be many different shapes
depending on the desired shape of the light output. The LED package
50 can comprise a meniscus holding feature for encapsulants that
are dispensing and is arranged to form a meniscus as a result of
surface tension between it and the encapsulant. This helps hold the
encapsulant in the hemispheric shape as shown until it is cured,
with the meniscus holding feature defining the outer boundary of
the encapsulant 56. Meniscus holding feature in the context of
dispensing encapsulant are discussed in U.S. Patent Application
Publication No. 2007/0228387, to Negley et al., entitled "Uniform
Emission Pattern LED Package," which is incorporated herein by
reference.
[0062] One suitable shape for the encapsulant 56 as shown is
hemispheric, with some examples of alternative shapes being
ellipsoid, bullet, flat, hex-shaped and square. Many different
materials can be used for the encapsulant including clear polymers,
silicones, plastics or epoxies, with a suitable materials being
compatible with the various encapsulant deposition methods
described above. Silicone is suitable for dispensing and molding
and provides suitable optical transmission properties. It can also
withstand some subsequent reflow processes and in many applications
exhibits an acceptable rate of degradation over time. It is
understood that the encapsulant 56 can also be textured to improve
light extraction or can contain materials such as phosphors or
scattering particles.
[0063] As discussed above, the encapsulant 56 in the packages
according to the present invention can comprise a thixotropic or
thickening agent/material 58 that helps allow the encapsulant to
hold its shape prior to and during curing. In the embodiments
according to the present invention, the thixotropic agent 58 can
have an index of refraction that is the same as or close to that of
the encapsulant material. In some embodiments, the thixotropic
agent 58 can have a refractive index that is within plus or minus
n=0.05 of the encapsulant. In still other embodiments the
refractive index can have a refractive index within plus or minus
n=0.04 of the encapsulant, while in other embodiments it can have a
refractive index within plus or minus n=0.02.
[0064] It is understood that the thixotropic agent 58 can be made
of many different materials or combination of materials. By way of
example, some package embodiments can have and encapsulant 56
comprising silicone, which can be optically clear to the light
emitted by the package LED, and can have a refractive index of
n.apprxeq.1.51. Fumed silica is often used as a thixotropic agent
58 and has a refractive index of n.apprxeq.1.46. As discussed
above, this difference in refractive index can cause scattering of
the LED light, which can result in a cloudy appearance of the
encapsulant and a reduction in package emission efficiency.
[0065] In embodiments according to the present invention, a
thixotropic agent 58 can be used having an index of refraction the
same as or closer to that of the encapsulant 56. This can comprise
a thixotropic agent 58 made of a single material having or an index
of refraction that more closely matches that of the encapsulant 56.
In other embodiments, more than one material can be combined to
provide a composite thixotropic agent 58 with the desired
refractive index. This composite can comprise one or more materials
having a refractive index higher than the encapsulant 56 that are
combined via suitable chemical reactions with materials having an
index of refraction lower than the encapsulant 56. This combination
can result in a thixotropic agent 58 having a refractive index that
is between the higher and lower refractive index materials and that
is the same as or close to the refractive index of the encapsulant
56, i.e. within one of the acceptable ranges discussed above. These
different materials can be combined and provided as a thixotropic
agent using known methods, such as fuming, spray or laser
pyrolysis, with the different materials provided in different
concentrations or percentages to provide the composite material
with the desired refractive index.
[0066] Many different materials can be used to form the thixotropic
agents according to the present invention, with one embodiment
comprising a composite material or mixture two or more oxides or
ceramics. In one embodiment, the thixotropic agent can comprise an
aluminosilicate combination of silica (SiO.sub.2) which has an
index of refraction of n.apprxeq.1.46 and alumina (Al.sub.2O.sub.3)
having an index of refraction of n.apprxeq.1.7. By combining these
materials in the appropriate percentage, the resulting composite
thixotropic agent can have an index of refraction the same or close
to that of the encapsulant, or n.apprxeq.1.51 for silicone. It is
understood that the other composite material can also be used such
as titania-silicate composites, or fumed aluminum oxide or titanium
oxide composites. For LED packages having higher index of
refraction encapsulants, the concentrations of materials in the
composites can be varied to allow for the refractive index of
thixotropic agent to match the encapsulant. For example, in the
thixotropic agent embodiment utilizing silica and alumina, greater
concentrations of alumina can be used to increase the refractive
index, or reduced amounts can be used to reduce the refractive
index.
[0067] Although the composite thixotropic agents are described
above combining two materials, it is understood that they can
comprise composites of many more than two materials. In these
embodiments one or more of the materials can have a refractive
index lower than the encapsulant's, and one or more can have a
refractive index higher that the encapsulant's. The materials can
be combined in different concentrations to achieve the desired
refractive index.
[0068] To allow for the desired rheology or thickness control, the
resulting composite thixotropic agent 58 should also comprise
particles having a large surface area and/or small particle size.
In some embodiments, the particles can comprise a surface area of
greater than 100 m.sup.2/g, while in other embodiments the surface
area can be greater than 300 m.sup.2/g. The particle size in some
embodiments can be less than 0.1 .mu.m, while in other embodiments
can be less than 0.5 .mu.m.
[0069] The shape and arrangement of the encapsulant 56 in the LED
package 50 is also easily adapted for use with secondary lens or
optics that can be included over the lens by the end user to
facilitate beam shaping. These secondary lenses are generally known
in the art, with many different ones being commercially available.
The encapsulant 56 can also have different features to diffuse or
scatter light, such as scattering particles or structures.
Particles made from different materials can be used such as
titanium dioxide, alumina, silicon carbide, gallium nitride, or
glass micro spheres, with the particles dispersed within the lens.
Alternatively, or in combination with the scattering particles, air
bubbles or an immiscible mixture of polymers having a different
index of refraction could be provided within the encapsulant or
structured on the encapsulant to provide diffusion. The scattering
particles or structures can be dispersed homogeneously throughout
the encapsulant 56 or can have different concentrations in
different areas of the encapsulant 56. In one embodiment, the
scattering particles can be in layers within the encapsulant.
[0070] The thixotropic agent can be provided in different
concentrations in the encapsulant, and in some embodiments the
thixotropic material can have a concentration of 20% or less, while
in other embodiments it can have a concentration of 10% or less. In
embodiments where the thermal or electrical properties of the
encapsulant are enhanced by the thixotropic agent, the
concentration can be even greater. In the embodiments described
above, the thixotropic agent can be provided with the same or
similar concentration throughout the encapsulant. In other
embodiments, the thixotropic agent can be arranged with different
concentrations in different areas of the encapsulant. In other
embodiments, different layers with different materials or
characteristics can be included on or part of the encapsulant.
[0071] FIG. 6 shows another embodiment of LED packages 60 that are
similar to the LED packages 50 shown in FIGS. 4 and 5, and show two
packages 60 prior to singulation. As above, each of the packages
comprises an LED chip 64 on the submount 62, and it is understood
that many more LED packages can be fabricated on a submount prior
to singulation. In this embodiment, first and second meniscus
forming features 66, 68 are included for formation of an
encapsulant with more than one layer or portion. A first
encapsulant portion 70 is dispensed or formed over the LED chip 64
and the surface of the submount 62, with the first meniscus feature
66 defining the outer edge of the first encapsulant portion 70.
First thixotropic agents 74 can be included in the first portion 70
to help the first portion 70 hold its hemispheric shape until it is
cured. Once it is cured a second hemispheric portion 72 can be
dispensed or molded over the first hemispheric portion 70, with the
second meniscus feature 68 defining the outer edge of the second
encapsulant portion 72. Second thixotropic agents 76 can also be
included in the second encapsulant portion 72, and it can then be
cured. In some embodiments, first and second thixotropic agents 74,
76 can be the same, and in other embodiments they can be
different.
[0072] This arrangement provides two layer (or portion)
encapsulants that allows for different materials and
characteristics in the different layers. For the LED packages 60,
the first encapsulant portion 70 can comprise optically clear
material with thixotropic agents having the same (or within the
ranges described above) refractive index as the encapsulant so that
the thixotropic agents to not degrade the clarity of the
encapsulant or the LED package's emission efficiency. The second
encapsulant portion 72 can comprise scattering properties, either
through the use of a second thixotropic agent 76 having a different
refractive index than the second portion 72, or by including
diffusive or scattering materials to the second portion 72. Many
different scattering particles can be used, including but not
limited to: [0073] kaolin; [0074] zinc oxide (ZnO); [0075] yttrium
oxide (Y.sub.2O.sub.3); [0076] titanium dioxide (TiO.sub.2); [0077]
barium sulfate (BaSO.sub.4); [0078] fused silica (SiO.sub.2);
[0079] aluminum nitride; [0080] glass beads; [0081] zirconium
dioxide (ZrO.sub.2); [0082] silicon carbide (SiC); [0083] tantalum
oxide (TaO.sub.5); [0084] silicon nitride (Si.sub.3N.sub.4) ;
[0085] niobium oxide (Nb.sub.2O.sub.5); [0086] boron nitride (BN);
or [0087] phosphor particles (e.g., YAG:Ce, BOSE)
[0088] More than one scattering material in various combinations of
materials or combinations of different forms of the same material
may be used to achieve a particular scattering effect.
[0089] FIG. 7 shows another embodiment of LED packages 80 according
to the present invention that comprises first and second
encapsulant portions 82, 84 similar to those in LED packages 60
shown in FIG. 6. In this embodiment, however, the first encapsulant
82 comprises scattering properties, either through thixotropic
agents having a refractive index different from the material
comprising the first encapsulant, or by including scattering
materials. The second encapsulant portion 84 can be optically clear
but can also include a thixotropic agent with the same refracting
index as the material comprising the second encapsulant portion
84.
[0090] FIG. 8 shows still another embodiment of an LED package 90
according to the present invention also having first and second
encapsulant portions 92, 94 that are similar to those shown in FIG.
6. In this embodiment, there is only a first meniscus feature 96
that defines the boundary of the first and second encapsulant
portions 92, 94. The first encapsulant portion 92 can be dispensed
and cured, with the second encapsulant portion 94 dispensed and
cured over the first. Like the LED packages 60, the first
encapsulant portion 92 can have thixotropic agents such that it is
essentially optically clear, while the second portion 94 can have
scattering properties. FIG. 9 shows still another embodiment LED
package 100 with a first and second encapsulant portions 102, 104
similar to those in FIG. 8, but with the first having scattering
properties and the second being optically clear as described
above.
[0091] It is understood that the LED packages according to the
present invention can have two-portion encapsulants arranged in
many different ways and can have more than two encapsulant
portions. It is also understood that the different layers can be
provided with different concentrations of thixotropic agents or
scattering material in different areas, and can comprise grading of
these agents or materials. It is also understood that curing steps
may or may not be used between application of the first and second
encapsulants.
[0092] In still other embodiments, the different encapsulant
portions can take any of the different shapes described above, and
can also take other non-hemispheric shapes to control the package
emission pattern. FIG. 10 shows one embodiment of a LED package
encapsulant 110 according to the present invention that comprises a
optically clear first encapsulant portion 112 having an oval shape,
with a scattering second encapsulant portion 114 having a circular
shape on the first encapsulant portion 112. It is understood that
in other embodiments, the first encapsulant portion 112 can have
scattering properties, while the second encapsulant portion 114 can
be optically clear. FIG. 11 shows a different embodiment of an LED
package 120 encapsulant comprising an optically clear first
encapsulant portion 122 having a substantially triangular shape at
its base, with a scattering second encapsulant portion 124 having a
circular shape at its base. Like above, in other embodiments the
scattering and clear properties can be reversed in the encapsulant
portions. These are only some of the multiple different shapes that
can be utilized in the encapsulants according to the present
invention, with others comprising ellipsoid, asymmetric-oblong,
etc.
[0093] In other embodiments the encapsulant or encapsulant portions
can comprise many other materials to provide various emission
characteristics. In some embodiments, one or more wavelength
converter materials can be provided in the encapsulant or different
encapsulant portions, such as those materials listed above.
[0094] As mentioned above, the present invention can be applied to
many different LED package architectures, including those having
reflective cups as shown in FIG. 2, and described above. The
arrangements described herein are only some of the many
applications that can utilize thixotropic agents described
herein.
[0095] Although the present invention has been described in detail
with reference to certain preferred configurations thereof, other
versions are possible. Therefore, the spirit and scope of the
invention should not be limited to the versions described
above.
* * * * *